EP1313065A1 - Method and apparatus for calibration of and imaging by means of an X ray imaging apparatus sensitive to the field of gravity - Google Patents

Abstract

Method for calibrating a gravitationally sensitive image recording unit has the following steps: determination of calibration data at a number of selected base positions of the support unit with which corresponding image data at the same positions can be corrected; determination of position data relative to gravity at the corresponding base positions; and assignment of position data to calibration data in a look-up table. <??>The invention also relates to a corresponding calibration device, a method for imaging using a device calibrated with the inventive method, a surgical navigation method, a data processing program and a device for implementing imagining or surgical navigation.

Description

The invention relates to a method and a device for calibrating an image recording device, the z. B. due to their installation in a protruding supporting structure is sensitive to gravity, i.e. in the case of mechanical deformations of the supporting structure affects the image geometry and in particular causes image distortion can be. The invention further relates to a method and an apparatus for Image generation with such an image recording device, as shown in particular in X-ray systems, e.g. B. those with a C-arm, is used. The invention relates finally an X-ray system with such devices.

Image distortions of the named origin occur particularly in the case of protruding and movable ones mechanical support structures for such image recording devices mechanical deformations. This problem is particularly evident in X-ray systems in which the image recording device is on a patient pivotable arm (C-arm) is attached, at one end of which the image recording device and at the other end is an X-ray source. by virtue of the relatively large dimensions of the arm and the relatively high weight of these two The C-arm can have different components depending on its rotational position and deform to such an extent that the captured images distort are.

WO 00/66971 describes a device for detecting the position and orientation a body with aids to determine the direction of the gravity vector, and a method of correcting x-ray images caused by gravity and the earth's magnetic field are distorted. This will correct a recorded and digitized x-ray image first the direction of the gravity vector and the position and orientation of the x-ray device and then detected by the deformation of the x-ray device and the local geomagnetic field caused distortions in the X-ray image finally determined with the help of these distortions in the stored x-ray image correct a computer. A major disadvantage here is that Image correction the device for determining the exact position and orientation of the X-ray device, and an additional computer may be required. This additional Equipment expenditure is disadvantageous, particularly in the case of mobile X-ray devices.

An object on which the invention is based is therefore a method and to specify a device with which a calibration of an image recording device with regard to image distortions of the type mentioned in the relatively simple and reliable Way is possible.

The invention is also intended to provide a method and a device for calibration be used for compensation in X-ray systems with a C-arm of image distortion caused by gravity mechanical deformations of the C-arm.

Furthermore, a method and a device for image generation are to be specified which / which is particularly suitable for use with an image recording device calibrated according to the invention and is optionally also designed for surgical navigation.

Finally, an X-ray system that is particularly suitable for mobile applications should also be used be created in which the above-mentioned gravitational influences without significant additional equipment, such as a position measuring system, compensated can be.

This object is achieved according to claim 1 with a method for calibrating a with a gravity-sensitive support structure held image acquisition device with the following steps: Determination of calibration data (calibration base points) in a plurality of selected positions of the support structure, with which one in the respective Position are corrected with the image recording device recorded image can; Determining position data of the in each case related to the direction of gravity Supporting structure in the selected positions; and assigning the position data to the Calibration data in a lookup table.

The object is achieved according to claim 5 with a device for performing the Method with the following features solved: a device for detecting the direction gravity relative to a position of the support structure; and an arithmetic and Storage unit for determining position data relating to the direction of gravity for a plurality of selected positions of the supporting structure and for creating and storing a look-up table with which the position data is used for the equalization image distortions caused by mechanical deformation in these positions suitable calibration data are assigned

The object is further achieved according to claim 6 with a method for image generation an image recording device held with a gravity-sensitive support structure solved with the following steps: taking a picture of an examination object in a selected position of the support structure; Find out the direction the gravity-related position data of the support structure in the selected position; Compare the determined position data with that stored in a look-up table Position data; Reading out calibration data, which at least essentially with the determined position data corresponding position data in the look-up table are stored; and equalizing the recorded image with the read calibration data.

Finally, the object according to claim 9 with a device for performing of the imaging process with the following features: a device for Detecting the direction of gravity relative to the support structure; a computing and Storage unit for determining position data relating to the direction of gravity the support structure, to compare this position data with that in a look-up table stored position data, for reading out calibration data, which under at least essentially matching the determined position data Position data are stored in the lookup table, as well as to equalize the recorded image of the examination subject or for distorting the image of an in the object under investigation with the read and if necessary, interpolated calibration data; and a display unit for Representation of the rectified image of the examination object or to represent the distorted image of the examination subject, in which the distorted image of the instrument is shown.

The main advantages of these solutions are that they provide much better image quality can be achieved that the handling is easy and that they are also used Application with mobile X-ray systems are suitable without the usual position measuring systems required are.

Finally, it can also correct distortions caused by a curved one Surface of the entrance window of the image intensifier arise.

The dependent claims contain advantageous developments of the invention.

In the simplest case, the calibration data can be recorded by recording a phantom object or according to claim 3 with the aid of a physical model be calculated.

The embodiment according to claim 4 is particularly suitable for pivoting support structures on.

Finally, the embodiment according to claim 7 is particularly for the surgical Suitable for navigation.

Fig 1 eine schematische Gesamtansicht eines Röntgensystems mit einer erfindungsgemäßen Vorrichtung; und

Fig 2 eine schematische Darstellung einer Anordnung zur Kalibrierung des Röntgensystems.

Further details, features and advantages of the invention result from the following description of preferred embodiments with reference to the drawing.

1 shows a schematic overall view of an X-ray system with a device according to the invention; and

2 shows a schematic representation of an arrangement for calibrating the x-ray system.

Figure 1 shows a mobile X-ray system with a C-arm 1, at one end one X-ray source 2 and an X-ray detector at its opposite end 3 is arranged with image intensifier. There is also a three-axis accelerometer on the C-arm 1 31 attached with which the direction of the gravitational field is measured relative to the C-arm 1. The C-arm 1 is pivotable on a holder 4 stored, which in turn is attached to a table 5. The table 5 is mobile and with Operating elements as well as supply and operating facilities for the X-ray system Mistake.

An examination object (patient) is placed between the source 2 and the detector 3, the C-arm 1 on the bracket 4 generally at least 180 degrees can be swiveled in order to optimally examine the subject to be examined To achieve range.

As already mentioned at the beginning, such a C-arm can by the weight of the X-ray source 2 and the X-ray detector 3 and the image intensifier be deformed so that captured images are distorted. For this reason, one is Carry out calibration with which depends on the swivel position of the C-arm Deformations are compensated and thus the different distortions caused by them Getting corrected.

The calibration takes place after the production of the X-ray system and, if necessary, in regular intervals (service intervals). The main steps of such The procedure will be explained below. Alternatively, however, there are also modifications this procedure or other procedures applicable.

First, as shown in Figure 2, a phantom object in the form of two parallel plates 32, 33 attached to the X-ray detector 3. To this end serve rods 34, with which the plates are also held in parallel. The first plate 32 is located directly in front of the entrance window of the detector 3 and has a plurality of circular areas or balls 32x provided on the grid points of an imaginary square Grid arranged and are opaque to X-rays. Between the first plate 32 and the X-ray source 2, the second plate 33 is at a distance for example about 37 centimeters from the first plate. The second plate is with one A plurality of circular surfaces of the same size and also impermeable to X-rays or balls 33x provided, however, on the circumference of a centered circle are arranged

The first plate 32 is used to determine the distortion parameters of a projection image, while the second plate 33 is used to determine the actual focus position, each for a variety of selected pivot positions of the C-arm 1. Die Distortion parameters and focal positions are identified as distortion records stored for each of the pivot positions of the C-arm 1.

In detail, the circular areas or balls 32x of the first plate 32 are placed on the detector 3 projected, recorded with a segmentation algorithm and the individual circular areas or balls 32x on the first plate, the positions of which are known. Thereby and by appropriate interpolation between the surfaces, the distortion parameters can then for "every" X-ray beam and therefore for every pixel in itself can be calculated in a known manner.

Furthermore, with the aid of the image projected onto the detector by the second plate 33 of the circle of circular surfaces 33x and the ratio between the diameters of this Circle and the projected circle, the focal position in a conventional manner calculated.

From the distortion data records recorded for each of the pivot positions of the C-arm 1 then calibration data (calibration base points) are calculated which are saved and with which by the distortions and focus shifts caused an error recorded in the pivotal position concerned Image can be corrected (equalized).

Furthermore, with the three-axis accelerometer 31 in each of these pivot positions of the C-arm 1 the gravitational field with respect to its direction relative to that C arm measured. From this direction, related position data of the C-arm, in particular its actual swivel angle or its rotational position is calculated.

These position data are finally determined together with those for this position Calibration data stored in a lookup table in an associated manner.

If for a sufficient number of swivel positions of the C-arm 1 calibration data as well as the position data determined and stored in the lookup table calibration of the X-ray system has been completed.

Alternatively, the lookup table could be based on a physical Model of the mechanical deformation of the C-arm in different positions as well as the calibration data required for correction are calculated.

For calibration, d. h to create and save the lookup table, is either provide a separate computing and storage unit, or the calibration by a corresponding data processing program with one in the concerned X-ray system existing computer unit made.

For the generation of images when examining a patient or another object the C-arm 1 is initially pivoted, as usual, into a position in which the person of interest The area is illuminated and a corresponding image is projected onto the image intensifier can be. When this position is reached, the picture is taken in a known manner. Furthermore, with the three-axis accelerometer 31, the gravitational field in terms of its direction relative to the C-arm. From this relative direction of the gravity field, the position data of the C-arm 1 are then again calculated and compared with the position data stored in the lookup table. When a matching or substantially matching entry can be found, the calibration data assigned to this entry are read out and corrected of the recorded image used in a manner known per se.

Alternatively, the position data of the C-arm during calibration and the Image generation, for example with an optical position measuring system (OPMS) can be calculated so that the accelerometer is not required. Furthermore, the accelerometer could also swing the C-arm tracking system attached to the position of the position data of the C-arm to calculate.

If there is insufficient agreement between the identified and those in the Position data stored in the table is found, the relevant calibration data be interpolated. For this purpose, the calibration data is as Calibration base points considered. Various methods can be used. The application of an approximate Delauney triangulation is explained here as an example.

It is an approximation of the triangulation because the calibration base points lie on a spherical surface for the sake of simplicity however, the base points formed by Delauney triangles are treated as planar triangles should. Before the interpolation, every base point must be on such planar triangles be projected.

The original (planar) process works as follows. It was from a set of too interpolating calibration construction points on a spherical surface. The Triangulation algorithm results in a set of non-intersecting planes Triangles, the corners of which are each formed by calibration base points, so that the entire area is covered with triangles.

Any intermediate point P (i.e. a calibration base point to be interpolated) can now be clearly assigned to one of the triangles. The corner points of this triangle represent the three calibration base points closest to point P. Consequently the base points are determined which are to be selected for the interpolation.

The Delauney triangulation is clear. It works for a two-dimensional plane Algorithm as follows: first, all of the set of calibration base points possible triangles. Those triangles whose corner points are collinear remain unconsidered. If the circle circumscribing a triangle contains other base points, the triangle is also disregarded. Only if these two cases are not given the triangle is used.

However, since the calibration base points are actually on a spherical surface, the Avoiding geometric distortions and other Delauney problems Triangulation adapted to a spherical interpolation and changed in the way (approximated) that the calibration base points into a three-dimensional Cartesian Coordinate system can be transformed. It remains coplanar for each triple Basis points, the corresponding triangle is not taken into account.

Instead of the circumscribing circle mentioned above, an enclosing sphere becomes is formed, and its radius is determined by the three-dimensional Euclidean distance from compared to any other calibration base point. One can demonstrate that this Criterion is equivalent to that of normal two-dimensional Delauney triangulation, if the base points lie on an ideal spherical surface. If the enclosing ball contains other base points, the triangle is disregarded. If not the triangle is applicable.

To simplify and accelerate the interpolation calculations, instead of the point P lying on the spherical surface, the projection P 'of the point P onto the plane Triangular area considered. This only leads to slight distortion effects with large triangles in the interpolation contributions.

When the triangulation is complete, the interpolation can be done through the planar triangles in a simple manner using center of gravity coordinates (barycentric Coordinates) can be calculated. A point P 'lying in the plane (C1, C2, C3) can can be described by its barycentric coordinates (B1, B2, B3). In the following assumed that a point P on the spherical surface for which the interpolation coefficient are to be calculated, projected onto each planar triangular surface to be taken into account (Point P ').

The barycentric coordinates each contain information about the relative Position of point P 'with respect to one of the sides of the triangle. For a corner point C1 of the triangle B1 is negative if the point P 'is beyond that defined by the corner points C2 and C3 is 0 if it is on this line and positive if it is on the line is on the same side of the line as the corner point C1.

In this way, the barycentric coordinates provide a simple criterion for the Find the appropriate interpolation triangle. Only if all values Bi are greater than 0 point P 'lies within the triangle.

Now that the barycentric coordinates have been determined, they can be used for the point P 'values to be interpolated by a simple linear combination and thus the interpolated calibration base point can be calculated, with which the recorded one Image is corrected or rectified.

For image generation and any interpolation that may be required is again either a separate computing and storage unit or a corresponding data processing program provided that with an existing in the X-ray system in question Computer unit is running

In addition, it should be noted that in addition to the image rectification of a recorded The principle according to the invention is also shown, for example, for surgical navigation is applicable. It is not primarily a question of taking a picture equalize, but as accurately as possible the position of an inserted in the patient Instrument (for example, a catheter) to determine and with a corresponding Fade in the image processing system into the captured image.

On the one hand, an X-ray image of the area to be examined is obtained in the usual way of a patient without equalizing it. On the other hand, the current one Position of the inserted instrument using a known method or Position measuring device (for example using a small transmitter or an inductor the tip of the instrument). This position is then under (reverse) Application of the look-up table showing the distortion properties of the X-ray machine describes, distorted. In other words, this means that the (virtual) image of the imported Instruments according to those saved in the form of the calibration data Imaging properties of the X-ray device is distorted. This distorted picture then becomes using a corresponding display unit in the recorded (distorted) X-ray image displayed so that the instrument is now in the right place in the X-ray image appears.

This has the advantage that even with ongoing tracking or ongoing updating the (generally managed) instrument only one X-ray must be taken. In addition, the instrument still needs not to be introduced so that the x-ray exposure is not impaired can be.

It is not necessary to correct the image, since only the actual position of the Instrumentes relative to the object to be examined is important.

Claims (10)

Method for calibrating an image recording device held with a gravity-sensitive support structure with the following steps: Determining calibration data in a plurality of selected positions of the support structure, with which an image recorded in the respective position with the image recording device can be rectified; Determining position data of the support structure in the selected positions, each relating to the direction of gravity; and Assign the position data to the calibration data in a look-up table. Method according to claim 1,
characterized in that the calibration data are determined with the aid of a recorded phantom object. Method according to claim 1,
characterized in that the calibration data are calculated using a physical model of the mechanical deformation of the support structure. Method according to claim 1,
characterized in that the position data are the swivel angle of the support structure. Device for carrying out the method according to one of claims 1 to 4, with means (31) for sensing the direction of gravity relative to a selected position of the support structure; and a computing and storage unit for determining positional data relating to the direction of gravity for a plurality of selected positions of the supporting structure and for creating and storing a look-up table with which the positional data in each case the calibration data suitable for correcting image distortions caused by mechanical deformations in these positions assigned. Method for image generation with an image recording device held with a gravity-sensitive support structure, which is calibrated in particular according to one of claims 1 to 4, with the following steps: Taking an image of an examination object in a selected position of the supporting structure; Determining position data relating to the direction of gravity of the supporting structure in the selected position; Comparing the determined position data with the position data stored in a look-up table; Reading out calibration data which are stored in the look-up table under position data which corresponds at least substantially with the determined position data; and Equalization of the recorded image with the read calibration data. A method for image generation, particularly in surgical navigation, with an image recording device, which is held with a gravity-sensitive support structure and which is calibrated in particular according to one of claims 1 to 4, with the following steps: taking an image of an examination object in a selected position of the support structure; Determining position data relating to the direction of gravity of the supporting structure in the selected position; Comparing the determined position data with the position data stored in a look-up table; Reading out calibration data which are stored in the look-up table under position data which corresponds at least substantially with the determined position data; Determining the position and recording a (virtual) image of an instrument inserted into the examination object; Calculating a virtual, distorted image of the instrument inserted into the examination object with the aid of the read-out calibration data; and Fade in the distorted image of the inserted instrument into the recorded image of the examination object. Data processing program with program code means for performing the steps of the method according to one of claims 1 to 4 or 6 or 7, if the program in running on a computer. Device for performing the method according to claim 6 or 7, with means (31) for detecting the direction of gravity relative to the support structure; a computing and storage unit for determining position data relating to the direction of gravity of the supporting structure, for comparing this position data with the position data stored in a look-up table, for reading out calibration data which are stored in the look-up table under position data which at least essentially matches the determined position data are, and for equalizing the recorded image of the examination object or for distorting the image of an instrument inserted into the examination object with the read-out and, if necessary, interpolated calibration data, and a display unit for displaying the rectified image of the examination subject or for displaying the distorted image of the examination subject, in which the distorted image of the instrument is superimposed. X-ray system with an image intensifier and a device according to Claim 5 or 9.

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